1. Field of the Invention
The present invention relates to a carrier for electrostatic latent image developer that is used in developing an electrostatic latent image formed, for example, by an electrophotographic or electrostatic recording process, a production method thereof, an electrostatic latent image developer, and an image-forming device.
2. Description of the Related Art
Processes of visualizing image information via forming an electrostatic latent image by an electrophotographic process or the like are currently used in a variety of fields. In the electrophotographic process, an image is visualized by forming an electrostatic latent image on the surface of a photoreceptor by charging and exposure, developing the electrostatic latent image into a toner image with an electrostatic latent image developer (hereinafter, sometimes referred to simply as “developer”) including toner, and then transferring and fixing the toner image. The developers currently used include two-component developers containing a toner for developing an electrostatic latent image (hereinafter, sometimes referred to simply as “toner”) and a carrier for electrostatic latent image developer (hereinafter, sometimes referred to simply as “carrier”) and one-component developers such as magnetic toners that are used alone. The two-component developers are used widely because of their superior controllability, because the carrier therein plays the roles, for example, of agitating, transporting and charging the developer and thus, the functions of the developer are separated.
Generally, carriers are broadly grouped into carriers having a resin-coated layer on the surface thereof and carriers having no resin-coated layer, but the resin-coated carriers are superior when various electrostatic properties and the lifetime of developer are considered, and thus various resin-coated carriers have been developed and commercialized.
Recently, printing machines for the electrophotographic process allowing ultrahigh-speed on-demand printing have been studied to replace the offset printing machines used for printing newspapers and direct mailings. In the electrophotographic process, developments are in progress aimed at coping with expansion in the width of paper and increasing practical printing volume by increasing speed. However, printing at high speed, for example, at a linear velocity of 1,000 mm/sec or more (output of about 400 sheets of A4 paper per minute) raises the stress applied to the developer, which is proportional to the square of the speed, to a level beyond comparison with that applied in low-speed desktop machines.
Generally, for the purpose of optimizing printing performance, a conductive material such as carbon black for adjustment of electric resistivity is used in the resin-coated layer on the carrier surface, but in high-speed color machines operating at a linear velocity of 1,000 mm/sec or more, the conductive material is often separated, alone or together with the coating resin, from the carrier by the stress applied to the developer, causing the problem of contamination of the toner by the conductive material. Particlarly when color toners are used, the influence is amplified because of deterioration in image color reproducibility.
As for maintenance, it is necessary to change the developer at a certain interval even if the speed is raised, and accordingly, longer lifetime of developer is demanded especially for high-speed machines. Thus, it is necessary to prevent separation of the conductive material such as carbon black from the coating material of a carrier in high-speed color printers (printing machines) and obtain durability equivalent to or greater than that of monochrome machines.
For prevention of the separation of carbon black form the carrier for color toners, a method has been proposed of coating a carbon black-containing coating agent on the magnetic core (core material) of a carrier and then additionally coating a non-carbon black-containing coating agent thereon as a surface-coat layer (e.g., Japanese Patent Application Laid-Open (JP-A) No. 8-179570, the disclosure of which is incorporated by reference herein). However, the method prohibits control of the hardness of the surface and internal coat layers, the durability of the surface-coat layer is not satisfactory, and further, although separation of carbon black is not observed at an early stage, the coating agent is scraped off during continuous use, causing noticeable contamination by carbon black.
Alternatively, a bilayer coat including an internal coat layer of styrene resin or a styrene-acrylic resin has been proposed (e.g., JP-A No. 3-73968, the disclosure of which is incorporated by reference herein). In this proposal, disclosed is an example of using a styrene or acrylic resin for the internal coat layer and a silicone resin for the surface-side coat layer, because when a styrene or acrylic resin having a low surface tension and a low thermal decomposition temperature is used for the surface-side coat layer, the developing performance is deteriorated by the contamination due to filming of the toner. However, when exposed to a temperature of 200° C. or higher for hardening the silicone resin, the acrylic resin decomposes, resulting in separation of the coat layer and prohibiting production of a desirable carrier.
As for a fixing system, it is particularly desirable to avoid paper jamming and generation of paper powder formed by the friction between a device and paper in high-speed machines. Thus, non-contact fixing processes in which contact with medium is limited and paper jamming is caused extremely rarely are desirable. Generally, among such processes, oven fixing and flash fusing (light fixing) are promising. In particular, printing machines using a flash fusing process using light are attracting attention, because they give a high-quality image, are compatible with various media, allow quick start without standby power, and are higher in reliability, for example, in resistance to paper jamming.
Accordingly, it is important to stabilize developer in particular when a flash fusing process is employed, and thus, stabilization of the properties of developer, for example, increase in the durability of carrier, is an important issue.